Wow, it's been over a year since I last logged in here. And it's been a month since this thread has been posted on, so this is a little sudden, but I was thinking of a team for Pokemon Emerald designed solely for the Battle Frontier, yet only using Pokemon and moves that can be obtained in the game without trading or hacking. Here's what I got:
Latias
-Dragon Claw
-Psychic
-Fly (To make my life easier)
-Surf
Nature: Any nature that lowers attack

I'd be happy to give ya some team advice, but I gotta know the EV spreads first. :P Mind edittin' those in?

Usually I don't bother with EVs on in-game teams, but because this is a Battle Frontier team I was considering trying to keep certain team members away from Pokemon that give certain EVs. I don't feel like bothering with anything past that. Heck, I'm so lazy I neglected breeding Lileep to get Recover. (And when I say "usually" I mean "never.")

Since this Latias is all 'bout offense, you're most definitely better off using a Latios. Also, I would highly recommend you drop Surf and Fly for BoltBeam. Since ya won't catch Lati@s until post-story anyways, it'd be really wasteful to chuck Fly onto it and hurt the moveset like that. You could also replace Dragon Claw/Psychic with Recover if ya wanted, since Ice/Dragon moves have rather redundant coverage and Psychic isn't a good offensive type anyways.

Shadow Ball isn't all-that useful for Metagross (although it does snuff out Gengar, which I'll get into detail later why that's important), so if ya wanted, you could throw Explosion over the move instead. Explosion is a fantastic move overall, but when the game is 3v3, that Explosion coming from somethin' as strong as Metagross can easily be the difference between a win and a loss. So yeah, that's somethin' to consider.

Oh, Raichu... How terrible you are. Is there any particular reason why you're using it? It gives ya three Pokes weak to Ground, and it's just a bad Poke overall. Even moreso against CPUs. If ya don't have any good reason, I'd really recommend replacing it with anything, even if it just has better stats lol. It doesn't really do anythin' for your team after all.

Stall isn't too effective in the Battle Frontier, and Ingrain will bite ya in the ass more than it'l help ya more often than not. I know ya said you were too lazy to breed for Recover, but... Be lazy over somethin' less important, yeah? :P Recover is actually useful after all, unlike Ingrain.

Fire, Fighting, and Ground is literally the most redundant three types of coverage you can put together, so you can do without that Earthquake. Rock Slide wouldn't be a bad move to put over it. Blaze Kick's inaccuracy and lower power than Flamethrower makes it a worse move, so ya definitely don't want Blaze Kick. You could throw Fire Blast or Flamethrower on this moveset as your STAB Fire move of choice, either or.

Walrein isn't really that useful for full-on offense... A Pokemon like Sharpedo, Gyarados, or Starmie would be more suited to such a moveset. Though Starmie would add another weakness to Ghost, in regards to this particular team.

As I've been mentioning, Gengar isn't the absolute bane of this team, but it's a very valid threat. It outspeeds your whole team (since ya won't be EVing Lati@s), and as your current Poke selection is, you don't have a single resist to Ghost. Gengar has always been a valid threat in every generation, and having a weakness to it can be VERY problematic.

Also, as I mentioned when reviewing Raichu, your team's weakness to Ground is kinda undesirable. It's not as important when you do 3v3s/4v4s and you can leave out members of your team, but since Raichu isn't a good Poke anyways, you might want to consider bringing somethin' onto the team that is better. Preferably a Poke without a Ground weakness.

I agree to delete Dragon Claw, Fly and Surf in favor of Recover, Ice Beam and Thunderbolt. I will keep Psychic just so I have a strong STAB move.
Metagross I won't change. I'll just keep Shadow Ball (it could be worse, I originally had Psychic on the thing.)
I dunno what to replace Raichu with. Maybe Ninjask?
I will agree to not be a lazy bastard and breed Recover onto Cradily.
I agree to replace Blaze Kick and Earthquake with Flamethrower and Rock Slide.
I agree to replace Walrein with Sharpedo, which allowed me to keep the moveset the same, except I replaced Body Slam with Crunch.
Now to look up how to get Recover on a Lileep.

While it's probably not the optimal choice, you could throw Heracross on the team just because. It resists Bug, Ground, and Fighting which are some of your team's problem types, and it's a good Poke overall. For the same reasons, you could chuck a Ghost type like Gengar (keep it away from other Gengar, obviously) or Dusclops, although only the former is immune to Ground. Dusclops is kinda meh either way, though...